The aim of this study is the better understanding of the dynamic relationship between airway diameters and lung volume or transpulmonary pressure. Most previous studies have dealt only with static behavior of airways. There is reason, based on the rat dependent behavior of smooth muscle and of airway and lung hysteresis, to suspect that dynamic airway behavior may be different from the behavior of airways during static maneuvers. The first stage of the project has been the development of a suitable diameter measuring device which can be inserted into intrathoracic airways of anesthetized dogs. A prototype device was constructed using a single strain gauge. In an initial study, the device performed well, measuring the diameter of the intrathoracic trachea. Pressure diameter (P-D) and volume diameter (V-D) plots will be recorded during tidal breathing. The P-D and V-D characteristics of airways may depend on breathing pattern and on bronchomotor tone. Breathing pattern will be changed by altering tidal volume, respiratory frequency and mean lung volume in artificially ventilated animals. Bronchomotor tone will be altered by aerosol administration of bronchoactive drugs and by electrical vagal stimulation. The results will be evaluated with reference to the hysteresis of parenchyma and airways, the interdependence or independence of airways and parenchyma, and the implications of the findings to airway diameter and airway conductance in normal physiology and disease states.